Conventional telephoto zoom lenses are generally configured in four groups which are plus-minus-minus-plus or plus-minus-plus-plus and usually have a fixed aperture stop in the rear positive group which is also fixed in relation to the image planes. In these types of lenses, the size of the front group and therefore, the maximum diameter of the lens, is determined by the size of the entrance pupil at the long end of the zoom lens range. This requires that the smallest diameter the front group may have is the diameter of the entrance pupil at the long end of the range. Often, the diameter of the front group should be larger than that of the entrance pupil to satisfy relative illumination requirements at the edges of the images format. Therefore, if the lens is required to have a large relative aperture, the diameter of the front group is large, making the lens impractical and expensive for the high volume consumer market. Additionally, the large barrel diameter may be found objectionable by the user.
More modern forms of telephoto zoom lenses have the aperture stop and/or the rear group both moving during a zooming operation. These configurations usually result in a smaller physical size of the lens since the lens then has a variable relative aperture number. With the variable aperture, the lens is usually slower at the longer equivalent focal length (EFL), resulting in a smaller entrance pupil at that end. These forms of lenses generally require a more elaborate motion of the groups that move with zooming, with the front group possibly moving for both zooming and focusing. As a result, the mechanical configuration of the lens becomes more complicated than that of the conventional forms, and the lens more difficult to manufacture.
The conventional approach to the design of a zoom lens is to minimize lens group powers and maximize zooming group travel. That, in turn, means that the centration and the tilt tolerances have to be controlled quite accurately over relatively long travel distances. And since the motion of every zoom group is represented by a quadratic function, the long travel can often require zoom cams which must change the acceleration and the direction of motion of zooming groups. This may often result in designs which are difficult or simply impossible to physically realize.
The present lens provides a three group zoom lens of a plus-minus-plus configuration, wherein the front positive group is stationary with zooming and moves only for focusing, and the second negative group and the third positive group move in opposite directions relative to each other for zooming.